Journal of Alloys and Compounds 602 (2014) 58–65

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Journal of Alloys and Compounds

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Experimental investigation and thermodynamic calculationof the Al–Si–Ti system in Al-rich corner

http://dx.doi.org/10.1016/j.jallcom.2014.02.1070925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +86 21 56333853.E-mail address: kcc126@126.com (K.-C. Chou).

Qun Luo, Qian Li, Jie-Yu Zhang, Shuang-Lin Chen, Kuo-Chih Chou ⇑Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, China

a r t i c l e i n f o

Article history:Received 8 October 2013Received in revised form 14 November 2013Accepted 18 February 2014Available online 26 February 2014

Keywords:IntermetallicsPhase diagramsScanning electron microscopyX-ray diffractionThermodynamic modeling

a b s t r a c t

The phase equilibria in Al-rich corner of the ternary Al–Si–Ti system were investigated using X-raydiffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). Theternary phase s2 (Ti33Al15Si52, ZrSi2-type) existed stably and equilibrated with TiAl3, fcc(Al) and Si. TheDSC results from room temperature to 1000 �C indicated that the endothermic peaks corresponded tothe quasi-peritectic reaction, liquid + TiAl3 ? s2 + fcc(Al), at 595 �C and the eutectic reaction, liquid ?s2 + fcc(Al) + Si, at 575 �C. Based on the experimental results, a set of self-consistent thermodynamicdescription of the Al–Si–Ti system was obtained. The calculated isothermal section at 450, 500, 550and 700 �C and the invariant reactions agreed well with the experimental data.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Owing to its high wear resistance, low thermal-expansioncoefficient, good corrosion resistance and excellent mechanicalproperties at a wide range of temperatures, the casting Al–Si alloysare widely used in military, automobile and general engineeringindustry [1–3]. The application of grain refiner and Ti containingmaster alloys for increasing wear resistance and mechanicalproperties [4,5] may result in the formation of Ti–Al based inter-metallics [6]. However, limited work has been reported on phaseequilibria in the Al-rich corner of the Al–Si–Ti system. The avail-able data on the ternary compounds were contradictory [7,9,13],which were summarized as follows.

Raman and Schubert [7] investigated the isothermal section ofthe Al–Si–Ti system at 700 �C and discovered two ternary interme-diate phases, Ti7Al5Si12 and Ti2AlSi3. The crystal structure ofTi2AlSi3 is identical to that of Ti(AlxSi1�x)2 (s2, ZrSi2-type) givenby Brukl et al. [8]. Mondolfo [9] suggested that no ternary phaseformed in the Al corner of the Al–Si–Ti system and the recordedphases Ti7Al5Si12, Ti2AlSi3 and TiAlSi2 were the solid solution ofAl in TiSi2. Zakharov et al. [10,11] determined a ternary intermedi-ate phase Ti2Al3Si2 with a relatively narrow composition region42.5–43.3 wt.% Ti, 22.1–24.8 wt.% Si, and remainder Daud andSaheb [12] reported a ternary compound AlSi2Ti whose crystalline

structure was different from those of TiAl3 (DO22-type) and TiSi2

(C54-type) compounds and the existence of other ternary com-pounds were not observed in their study. Liu et al. [13] reinvesti-gated the phase equilibria of the Al–Si–Ti system over the entirecomposition range. The results showed that the ternary phase s2

(ZrSi2-type orthorhombic) formed incongruently at 1338 �C withcomposition ranging from Ti33Al21Si46 to Ti33Al8Si59 at 1000 �Cand the ternary phase s1 (Zr3Al4Si5-type tetragonal) melts incon-gruently between 953 and 1000 �C with the composition of Ti33.4-

Al8Si58.6. The phase equilibria in the Ti–Al–Si system werestudied in the temperature range of 700–1000 �C using ternarydiffusion couples, in which two ternary intermetallic compoundsTi3Al2Si5 and Ti3AlSi5 were observed [14]. Gao et al. [15] found thatthe increase of Si addition in Al–xSi�2Ti alloys led to a phase evo-lution from Ti(Al1�xSix)3 to Ti7Al5Si12 and the limited solubility of Siin Ti(Al1�xSix)3 lay between 12.5 and 18.75 at.%. The results of Sa-heb et al. [4] showed that the addition of Ti led to the precipitationof TiAl3 phase which dissolved Si up to 13.12 at.%. From the exper-iment results above, it can be seen that the composition of ternarycompounds and the phase equilibria in Al-rich corner of Al–Si–Tisystem are still unconfirmed. However, it is very significant toclarify this issue for designing the casting Al–Si alloys.

With regard to thermodynamic calculation, the CALPHADassessment of the Ti-rich part of the diagram (<25 at.% Al and<5 at.% Si) between 700 and 1200 �C was carried out by Azevedoand Flower [16,17]. Gröbner et al. [18] provided a thermodynamicdescription of Al–Si–Ti system to understand the mechanism of

Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65 59

grain refining in this ternary alloys. Based on the experimental datafrom Al–10 wt.% Si/Ti diffusion couple, Li et al. [19] reassessed theAl-rich corner of Al–Si–Ti system. In this study, the phase equilibriain the Al-rich corner were investigated experimentally, and aconsistent thermodynamic description of Al–Si–Ti system was as-sessed. We hope to obtain the fundamental knowledge on thephase relationship in this system for designing the target materialwith desired properties.

2. Experimental procedures

Three Al–Si–Ti alloys were prepared in a medium frequency induction furnaceunder high purity argon atmosphere (99.999%). Owing to the insulation and highmelting point of Si, the Al–30 wt.% Si alloy (99.99%) was used as raw material.The weighted Al–30 wt.% Si blocks, Al blocks (99.99%) and Ti blocks (99.995%) weremelted in the furnace. The ingots were all re-melted for five times to ensure theirhomogeneity. Small strips of 4 mm � 4 mm � 20 mm were wire electro-dischargemachined from the ingots and sealed respectively in evacuated quartz capsuleswith atmospheric pressure at 10�2–10�1 Pa. Then the samples were annealed at450, 500, 550 and 700 �C for 310–3360 h (hrs), and finally quenched in water.

The actual composition of each alloy was measured by inductively coupledplasma atomic emission spectrometry (ICP). XRD analysis was performed on thesample using a D/MAX 2500 diffractometer with Cu Ka radiation. The microstruc-ture and composition of phases in the samples were examined by JSM-6700F SEMequipped with energy dispersive X-ray spectrometer (EDS, semi-quantitativeanalysis in this work). The samples annealed at 550 �C for 1128 h were studiedby NETZSCH 404C DSC under argon atmosphere at the flowing rate of 50 mL/minand the heating rate of 10 �C/min.

Fig. 1. The BSE images of the samples #1–#3 anne

3. Results and discussions

Fig. 1 shows the backscattered electron (BSE) images of thesamples #1–#3 annealed at 550 and 700 �C. Fig. 2 shows theXRD patterns of the three samples, from which it can be seen thata three-phase equilibrium of fcc(Al) + TiAl3 + s2 exists in sample #1in the temperature range of 450–550 �C, and a two-phase equilib-rium TiAl3 + fcc(Al) is observed at 700 �C. Small amount of s2 phasein sample #1 presents at the edge of TiAl3. At 700 �C, the liquidphase presents in all samples and the diffusion of components be-come faster. It is clear that the microstructures of samples becomeobviously coarser at 700 �C than that at 550 �C, as shown in Fig. 1.No amorphous phase found in the XRD patterns, which indicatesthat the liquid phase is not preserved after quenching. The contentof Si in fcc(Al) phase of sample #2 greatly exceeds its saturated so-lid solution. Therefore, the liquid should transform to the supersat-urated fcc(Al) phase during cooling process.

The stable phases observed in sample #2 are s2, fcc(Al) and Si at450–550 �C, and since fcc(Al) phase would melt at 700 �C, the ob-served phases should be s2, liquid and Si. The s2 and TiAl3 phaseseparate completely in sample #3 after annealing at 700 �C for310 h because of the fast diffusion rate in liquid. The XRD resultof sample #3 indicates the three-phase equilibrium of fcc(Al) +TiAl3 + s2. The phase compositions determined by XRD are consis-tent with that of EDS. The comparison of phase compositionsidentified by XRD and EDS for different alloys at different

aled at 550 �C for 3168 h and 700 �C for 310 h.

Fig. 2. The XRD patterns of cast and annealed samples.

60 Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65

temperatures is summarized in Table 1. It should be noticed thatthere is merely one ternary phase, s2, appearing at the Al-rich re-gion, which located at 33–35 at.% Ti, 8–18 at.% Al, 47–59 at.% Si.In other words, no other ternary phase is observed in those sam-ples. The XRD results indicate that the crystal structure of s2 isorthorhombic and of ZrSi2-type, which is different from TiAl3

(DO22-type) and TiSi2 (C54-type).

The phase transition temperature was determined by DSC fromroom temperature to 1000 �C. Fig. 3(a) is the graph of DSC heatingcurves of samples. The endothermic peaks shown in curves of sam-ples #2 and #3 indicate that there are two phase transitions at575 �C and 595 �C, respectively. Fig. 3(b) is the enlarged graph ofDSC curve of sample #1, which shows that two small endothermicpeaks present at 595 �C and 608 �C. In order to reveal the types ofphase transition and solidification procedure, the microstructure ofcast samples needs to be determined. Fig. 4 shows the BSE imagesof the cast samples #1–#3. With regard to sample #1, the TiAl3

phase is the primary phase and the surrounded fcc(Al) phase formssubsequently. The bright lamellate s2 in cast sample #2 randomlyoccur and traverse the Si particles, so the s2 should be the primaryphase. Then the Si particles form and finally the remainder liquidtransforms into fcc(Al) + Si eutectic microstructure through aeutectic reaction. The presence of irregular Ti5Si4 phases in castsample #3 was verified by XRD results. It is easy to know thatthe Ti5Si4 phase firstly crystallizes from liquid, and then the s2

and TiAl3 phases form simultaneously through a quasi-peritecticreaction, liquid + Ti5Si4 ? s2 + TiAl3. Therefore, the Ti5Si4 phase isenveloped in the s2 and TiAl3 phases (showed in the enlargedimage of sample #3). Finally, the residual liquid transforms intofcc(Al). The composition of each phase listed in Table 1 is averagedfor at least three measurements.

4. Thermodynamic modeling

4.1. Thermodynamic models

For the binary subsystems Al–Si [20], Al–Ti [21] and Si–Ti [22]the thermodynamic data sets were accepted as published. In thefollowing, the analytical expressions for the Gibbs energies of thephases in Al–Si–Ti system are presented.

The Gibbs energy function �G/i ðTÞ ¼ G/

i ðTÞ � HSERi for element i

(i = Al, Si and Ti) in all the phases detected (liquid, Al (FCC), Ti(HCP & BCC), and Si (Diamond)) is described as:

�G/i ðTÞ ¼ aþ bT þ cT ln T þ dT2 þ eT3 þ fT�1 þ gT7 þ hT�9 ð1Þ

where HSERi is the molar enthalpy of the component i in its stable

state at 298.15 K and 1 atm, and T is the absolute temperature.The Gibbs energies for Al, Si and Ti are taken from the SGTE compi-lation by Dinsdale [23].

The Gibbs energy function of the ternary system containing thephases of liquid, fcc(Al), hcp(Ti), bcc(Ti) and diamond(Si) are de-scribed using substitutional-solution model:

G/ ¼X3

i¼1

xi�G/

i þ RTX3

i¼1

xi ln xi þ Gex;/ ð2Þ

where xi is the mole fractions of Al, Si or Ti. Gex,/ is the excess Gibbsenergy of the phase, which defined as following equation in thiswork.

Gex;/ ¼ xAlxSi

Xm

k¼0

kL/ðAl;SiÞðxAl � xSiÞk þ xAlxTi

Xm

k¼0

kL/ðAl;TiÞðxAl � xTiÞk

þ xTixSi

Xm

k¼0

kL/ðTi;SiÞðxTi � xSiÞk

þ xAlxSixTiðxAl � 0LAl;Si;Ti þ xSi � 1LAl;Si;Ti þ xTi � 2LAl;Si;TiÞ ð3Þ

where kL/ðAl;SiÞ;

kL/ðAl;TiÞ and kL/

ðTi;SiÞ are the binary interaction parame-ters, the 0LAl;Si;Ti, 1LAl;Si;Ti and 2LAl;Si;Ti are the ternary interactionparameters.

The binary intermetallic phases are modeled based on its binarythermodynamic data sets. The solution of the third element inTi2Al5, TiAl2 and Ti3Si were small and can be negligible. Therefore,

Table 1The comparison of phase compositions identified by XRD and EDS for different alloys at different temperatures.

Samples #1 #2 #3

Actual composition at.% 78.2Al, 9.2Si, 14.3Ti 68.1Al, 28.0Si, 3.9Ti 57.7Al, 23.8Si, 18.5Ti

As cast XRD TiAl3 fcc(Al) Si s2 fcc(Al) Si Ti5Si4 fcc(Al) s2

EDS Al 65.6 Al 100.0 Al 14.0 Al 100.0 Si 100.0 Al 1.3 Al 100.0 Al 18.1at.% Si 7.4 Si 50.8 Si 41.7 Si 46.5

Ti 27.0 Ti 35.2 Ti 57.0 Ti 35.4

450 �C, XRD TiAl3 fcc(Al) s2 s2 fcc(Al) Si TiAl3 fcc(Al) s2

3360 h EDS Al 64.7 Al 98.8 Al 9.7 Al 7.7 Al 99.9 Al 0.6 Al 58.8 Al 98.0 Al 11.5at.% Si 8.9 Si 0.8 Si 57.3 Si 58.6 Ti 0.1 Si 99.4 Si 14.7 Si 1.3 Si 55.0

Ti 26.4 Ti 0.4 Ti 33.0 Ti 33.7 Ti 26.5 Ti 0.7 Ti 33.5

500 �C, XRD TiAl3 fcc(Al) s2 s2 fcc(Al) Si TiAl3 fcc(Al) s2

3168 h EDS Al 66.7 Al 98.4 Al 14.8 Al 10.9 Al 99.0 Al 1.0 Al 59.9 Al 100.0 Al 10.7at.% Si 6.2 Si 1.2 Si 50.5 Si 53.2 Si 1.0 Si 99.0 Si 12.7 Si 52.9

Ti 27.1 Ti 0.4 Ti 34.7 Ti 35.9 Ti 27.4 Ti 36.4

550 �C, XRD TiAl3 fcc(Al) s2 s2 fcc(Al) Si TiAl3 fcc(Al) s2

3168 h EDS Al 67.4 Al 99.0 Al 14.2 Al 10.9 Al 98.9 Si 100.0 Al 60.8 Al 99.1 Al 14.0at.% Si 6.2 Si 0.9 Si 51.9 Si 53.7 Si 1.1 Si 12.5 Si 0.9 Si 50.8

Ti 26.4 Ti 0.1 Ti 33.9 Ti 35.4 Ti 26.7 Ti 35.2

700 �C, XRD TiAl3 fcc(Al) s2 fcc(Al) Si TiAl3 fcc(Al) s2

310 h EDS Al 63.7 Al 98.9 Al 8.3 Al 88.6 Si 100.0 Al 60.6 Al 96.9 Al 12.6at.% Si 9.3 Si 1.1 Si 55.3 Si 11.4 Si 12.5 Si 3.1 Si 51.6

Ti 27.0 Ti 36.4 Ti 26.9 Ti 35.8

Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65 61

their binary Gibbs energy descriptions were used in the ternary cal-culation. The Ti2Al5, TiAl2 and Ti3Si, are modeled as stoichiometriccompounds. For the other binary intermetallic phases, TiAl, Ti3Al,

Fig. 3. (a) The DSC heating curves of alloys annealed at 550 �C for 1128 h and (b)the enlarged image of DSC curve of sample #1.

TiAl3, Ti5Si3, Ti5Si4, TiSi and TiSi2 are treated as compound energyformalism accounting for their defect structure and Si- or Al-solu-bility in those phases. The Gibbs energy function for an intermetal-lic phase with compound energy formalism is described as:

G/ ¼ ref G/ þ idG/ þ exG/ ð4Þ

where ref G/ is expressed in terms of compound energies and theirassociated sublattice species (Al, Si and Ti) concentrations yi, idG/

is the ideal mixing term which is assumed to be the random mixingof species in each sublattice. Taking TiAl3 as an example, the Si-solubility in TiAl3 is modeled as (Ti, Al)0.25(Ti, Al, Si)0.75 by extend-ing the binary model (Ti, Al)0.25(Ti, Al)0.75 in order to reflect theexperimental observation that Si mainly substituted Al on thesecond sublattice. The Gibbs energy per mole formula for TiAl3 isexpressed as:

GTiAl3 ¼ y0Aly00TiG

TiAl3Al:Ti þ y0Aly

00AlG

TiAl3Al:Al þ y0Aly

00SiG

TiAl3Al:Si þ y0Tiy

00TiG

TiAl3Ti:Ti

þ y0Tiy00AlG

TiAl3Ti:Al þ y0Tiy

00SiG

TiAl3Ti:Si

þ 0:25RT y0Al ln y0Al þ y0Ti ln y0Ti

� �

þ 0:75RT y00Al ln y00Al þ y00Ti ln y00Ti þ y00Si ln y00Si

� �

þ y0Tiy0Aly00Al

0LTiAl3Ti;Al:Al þ y0Tiy

0Aly00Ti

0LTiAl3Ti;Al:Ti þ y0Aly

00Aly00Ti

0LTiAl3Al:Al;Ti

þ y0Tiy00Aly00Ti

0LTiAl3Ti:Al;Ti

þ y0Tiy00Aly00Si

0LTiAl3Ti:Al;Si þ

1LTiAl3Ti:Al;Si y00Al � y00Si

� �h ið5Þ

in which y0Al and y0Ti are the site fractions of Al and Ti on the first sub-lattice, and y00Ti, y00Si and y00Al are the site fractions of Al, Si and Ti on the

second sublattice. The terms 0LTiAl3Ti;Al:Al,

0LTiAl3Ti;Al:Ti,

0LTiAl3Al:Al;Ti,

0LTiAl3Ti:Al;Ti,

0LTiAl3Ti:Al;Si

and 1LTiAl3Ti:Al;Si represent the interaction parameters that are functions

of temperature.The ternary phase s2 is modeled as a stoichiometric compound:

(Ti)0.333333(Al)0.15(Si)0.516667. The Gibbs energy per mole formula fors2 is expressed as:

Gs2 ¼ 0:333333�GHCPTi þ 0:15�GFCC

Al þ 0:516667�GDiamondSi þ aþ bT

ð6Þ

The models and parameters in the thermodynamic descriptionsof Al–Si–Ti system are listed in Table 2.

Fig. 4. The BSE images of the cast samples #1–#3.

Table 2The thermodynamic data sets of the Al–Si–Ti system.

Parameters a b Refs.

Liquid, substitutional-solution, (Al, Si, Ti)1

0LLiquidAl;Si

�11,655.93 0.92934 [20]

1LLiquidAl;Si

�2873.45 0.2945 [20]

2LLiquidAl;Si

2520.0 0 [20]

0LLiquidTi;Al

�112,570 41.11378 [21]

1LLiquidTi;Al

�7950.8 0 [21]

0LLiquidTi;Si

�255,852 21.8741 [22]

1LLiquidTi;Si

�25,025.3 2.00203 [22]

2LLiquidTi;Si

83,940.6 �6.71526 [22]

bcc(Ti), substitutional-solution, (Al, Si, Ti)1

0LbccðTiÞTi;Al

�129,396.7 40.0631 [21]

0LbccðTiÞTi;Si

�275,629 42.5094 [22]

1LbccðTiÞTi;Si

�25,025.3 2.00203 [22]

2LbccðTiÞTi;Si

83,940.6 �6.71526 [22]

Diamond(Si), substitutional-solution, (Al, Si, Ti)1

0LDiamondðSiÞAl;Si

111,417.70 �46.1392 [20]

0LDiamondðSiÞSi;Ti

0 80 [22]

fcc(Al), substitutional-solution, (Al, Si, Ti)1

0LfccðAlÞAl;Si

�3423.91 0.09584 [20]

0LfccðAlÞTi;Al

�124,270 43.8967 [21]

0LfccðAlÞTi;Si

�10,000 0 [22]

hcp(Ti), substitutional-solution, (Al, Si, Ti)1

0LhcpðTiÞTi;Al

�123,789 33.209 [21]

1LhcpðTiÞTi;Al

�16,034.9 12.1827 [21]

0LhcpðTiÞTi;Si

�302,731 69.0847 [22]

1LhcpðTiÞTi;Si

�25,025.3 2.00203 [22]

2LhcpðTiÞTi;Si

83,940.6 �6.71526 [22]

62 Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65

Table 2 (continued)

Parameters a b Refs.

TiAl, compound energy formalism, (Ti, Al, Si)0.5(Ti, Al, Si)0.5

0GTiAlAl:Al � �G

FCCAl

0 0 [21]

0GTiAlTi:Al � 0:5�GFCC

Ti � 0:5�GFCCAl

�37,445.1 16.7938 [21]

0GTiAlAl;Ti � 0:5�GFCC

Ti � 0:5�GFCCAl

37,445.1 �16.7938 [21]

0GTiAlTi:Ti � �G

FCCTi

0 0 [21]

0GTiAlSi:Si

50,000 0 This work

0GTiAlAl:Si

50,000 0 This work

0GTiAlSi:Al

50,000 0 This work0LTiAl

Al;Ti:Al = 0LTiAlAl;Ti:Ti

�63,490.1 7.7928 [21]

1LTiAlAl;Ti:Al = 1LTiAl

Al;Ti:Ti39,488.4 0 [21]

1LTiAlAl:Al;Ti = 0LTiAl

Ti:Al;Ti�28,311.6 10.8517 [21]

Ti2Al5, stoichiometric, (Ti)0.285714(Al)0.714286

0GTi2Al5Ti:Al � 0:285714�GFCC

Ti � 0:714286�GFCCAl

�40,495.4 9.52964 [21]

Ti3Al, compound energy formalism, (Ti, Al)0.75(Ti, Al)0.25

0GTi3AlAl:Al �

�GHCPAl

0 0 [21]

0GTi3AlTi:Al � 0:75�GHCP

Ti � 0:25�GHCPAl

�29,633.6 6.70801 [21]

0GTi3AlAl:Ti � 0:75�GHCP

Al � 0:25�GHCPTi

29,633.6 �6.70801 [21]

0GTi3AlTi:Ti �

�GHCPTi

0 0 [21]

0LTi3 AlAl;Ti:Al = 0LTi3Al

Al;Ti:Ti�71,277.9 25.47 [21]

TiAl2, stoichiometric, (Ti)0.333333(Al)0.666667

0GTiAl2Ti:Al � 0:333333�GFCC

Ti � 0:666667�GFCCAl

�43,858.4 11.0208 [21]

TiAl3, compound energy formalism, (Ti, Al)0.25(Ti, Si, Al)0.75

0GTiAl3Al:Al �

�GFCCAl

0 0 [21]

0GTiAl3Ti:Al � 0:25�GFCC

Ti � 0:75�GFCCAl

�40,349.6 10.3653 [21]

0GTiAl3Al:Ti � 0:25�GFCC

Al � 0:75�GFCCTi

40,349.6 �10.3653 [21]

0GTiAl3Ti:Ti �

�GFCCTi

0 0 [21]

0GTiAl3Ti:Si � 0:25�GFCC

Ti � 0:75�GDiamondSi

�40,000 10 This work

0GTiAl3Al:Si � 0:25�GFCC

Al � 0:75�GDiamondSi

40,000 10 This work

0LTiAl3Al;Ti:Al = 0LTiAl3

Al;Ti:Ti12,500 0 [21]

0LTiAl3Al:Al;Ti = 0LTiAl3

Ti:Al;Ti�61,849.5 36.5337 [21]

0LTiAl3Ti:Al;Si

9000 0 This work

1LTiAl3Ti:Al;Si

�143,000 0 This work

Ti3Si, stoichiometric, (Ti)0.75(Si)0.25

0GTi3SiTi:Si � 0:75�GHCP

Ti � 0:25�GDiamondSi

�50,000 0.79981 [22]

Ti5Si3, compound energy formalism, (Ti, Si)2(Ti, Si)3(Ti)0.25

0GTi5Si3Ti:Si:Ti � 5�GHCP

Ti � 3�GDiamondSi

�583,564 2.68514 [22]

0GTi5Si3Ti:Ti:Ti � 8�GHCP

Ti40,000 20 [22]

0GTi5Si3Si:Si:Ti � 5�GDiamond

Si � 3�GHCPTi

�206,191 16.4953 [22]

0GTi5Si3Si:Ti:Ti � 2�GDiamond

Si � 6�GHCPTi

417,373 33.8102 [22]

0LTi5 Si3Ti;Si:Ti:Ti = 0LTi5Si3

Ti;Si:Si:Ti�500,000 40 [22]

0LTi5 Si3Ti:Si;Ti:Ti = 0LTi5Si3

Si:Si;Ti:Ti43,024.3 �3.44194 [22]

Ti5Si4, compound energy formalism, (Ti, Al)0.555556(Si, Al)0.444444

0GTi5Si4Ti:Si � 0:555556�GHCP

Ti � 0:444444�GDiamondSi

�79,000 2.48595 [22]

0GTi5Si4Ti:Al � 0:555556�GHCP

Ti � 0:444444�GFCCAl

50,000 0 This work

0GTi5Si4Al:Si � 0:555556�GFCC

Al � 0:444444�GDiamondSi

10,000 0 This work

0GTi5Si4Al:Al �

�GFCCAl

10,000 0 This work

TiSi, compound energy formalism, (Ti, Al)0.5(Si, Al)0.5

0GTiSiTi:Si � 0:5�GHCP

Ti � 0:5�GDiamondSi

�77,530.9 3.81725 [22]0GTiSi

Ti:Al � 0:5�GHCPTi � 0:5�GFCC

Al�27,445.1 16.7938 [22]

0GTiSiAl:Si � 0:5�GDiamond

Si � 0:5�GFCCAl

10,000 0 This work0GTiSi

Al:Al � �GFCCAl

10,000 0 This work

TiSi2, compound energy formalism, (Ti)0.333333(Si, Al)0.666667

0GTiSi2Ti:Si � 0:333333�GHCP

Ti � 0:666667�GDiamondSi

�58,346.15 1.516 [22]

0GTiSi2Ti:Al � 0:333333�GHCP

Ti � 0:666667�GFCCAl

�33,858.4 11.0208 This work

s2, stoichiometric, (Ti)0.333333(Al)0.15(Si)0.516667

0Gs2Ti:Al:Si � 0:333333�GHCP

Ti � 0:15�GFCCAl þ 0:516667�GDiamond

Si�73,150 4.54545 This work

Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65 63

Fig. 5. The isothermal sections of the Al-rich corner of Al–Si–Ti system at 450, 500, 550 and 700 �C.

64 Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65

4.2. Thermodynamic calculations of the Al–Si–Ti system

Fig. 5 shows the comparison between experimental points andthe calculated isothermal sections at 450, 500, 550 and 700 �C,respectively. It can be seen that the calculated results agree wellwith the experimental data. The experimental results of annealedsamples #1 and #3 indicate the three-phase equilibriumTiAl3 + fcc(Al) + s2 stabilizing at 450–550 �C and the two-phaseequilibrium TiAl3 + liquid at 700 �C. It signifies that at least onephase transition happened between 550 and 700 �C. The DSCcurves show the phase transition temperatures are 595 �C and

Fig. 6. The isopleths of Al–Si–Ti system with (a) 1

608 �C for sample #1 and 595 �C for sample #3. The isopleths ofAl–Si–Ti system with 14.0 at.% Ti (sample #1) and 18.5 at.% Ti(sample #3) are calculated through Pandat software and showingin Fig. 6(a) and (c), respectively. A quasi-peritectic reaction,liquid + TiAl3 ? s2 + fcc(Al) is presented at 604 �C, of whichtemperature is close to that of endothermic peaks at 595 �C. Withregard to sample #1, the line calculation result indicates that twophase transitions, fcc(Al) + s2 + TiAl3 ? fcc(Al) + TiAl3 (595 �C) andfcc(Al) + TiAl3 ? TiAl3 + liquid (605 �C), happened near the temper-atures shown in the DSC curves (595 and 608 �C). The phases ofannealed sample #2 are fcc(Al) + s2 + Si at 450–550 �C and

4.0 at.% Ti, (b) 3.9 at.% Ti and (c) 18.5 at.% Ti.

Q. Luo et al. / Journal of Alloys and Compounds 602 (2014) 58–65 65

liquid + s2 + Si at 700 �C, respectively. The calculated isopleth at3.9 at.% Ti in Fig. 6(b) shows that a eutectic reaction liquid ? s2 +fcc(Al) + Si presents at 577 �C, which agrees well with the temper-ature of 575 �C determined by DSC.

5. Conclusions

The Al-rich corner of Al–Si–Ti system has been investigatedexperimentally and thermodynamically. The phase equilibria inthe Al-rich corner of Al–Si–Ti system were determined using equil-ibrated alloys with methods of SEM, EDS and XRD. There is only oneternary phase, s2, existing. Two three-phase equilibria (TiAl3 +fcc(Al) + s2 and fcc(Al) + s2 + Si) present at 450–550 �C and becomeTiAl3 + liquid + s2 and liquid + s2 + Si at 700 �C, respectively. TheDSC curves of samples indicate that two endothermic peaks oc-curred at 595 �C and 575 �C corresponding to the quasi-peritecticreaction of liquid + TiAl3 ? s2 + fcc(Al) and eutectic reaction ofliquid ? s2 + fcc(Al) + Si, respectively. A set of self-consistent ther-modynamic description of the Al–Si–Ti system is obtained byaccessing the phase equilibrium data and phase transition temper-atures from DSC. What is more, the calculations are compared withthe experimental results and a good agreement is reached.

Acknowledgements

The authors thank Instrumental Analysis and Research Centerof Shanghai University for their support of materials testing and

research. This work was financially supported by the NationalNatural Science Foundation of China (51074103 and 51222402).

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